cmd/compile: use bufio.Reader directly in lexer

Removes an intermediate layer of functions that was clogging up a
corner of the compiler's profile graph.

I can't measure a performance improvement running a large build
like jujud, but the profile reports less total time spent in
gc.(*lexer).getr.

Change-Id: I3000585cfcb0f9729d3a3859e9023690a6528591
Reviewed-on: https://go-review.googlesource.com/20565
Reviewed-by: Robert Griesemer <gri@golang.org>
Run-TryBot: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/compile/internal/gc/export.go

@@ -5,6 +5,7 @@
 package gc
 
 import (
+	"bufio"
 	"bytes"
 	"cmd/internal/obj"
 	"fmt"
@@ -387,7 +388,7 @@ func dumpexport() {
 			pkgMap = make(map[string]*Pkg)
 			pkgs = nil
 			importpkg = mkpkg("")
-			Import(obj.Binitr(&copy)) // must not die
+			Import(bufio.NewReader(&copy)) // must not die
 			importpkg = nil
 			pkgs = savedPkgs
 			pkgMap = savedPkgMap